1. Field of the Invention
The present invention relates generally to a thin type permanent magnet generator, and more particularly to a thin type permanent magnet generator that can be incorporated in a diskette designed to be used by inserting into a floppy disk drive, and when a memory, such as a magnetic card, is installed in the diskette, serves as a power source for the memory to write and read data in the memory.
2. Description of the Related Art
Medical information, such as personal medical checkup results, is stored in an IC card, from which medical institutions can retrieve such information for use in the treatment of a patient, and can store in the card the contents of his case records after some medical treatments. Plans are also now under way to use IC cards as electronic money. Under the plans, data on a customer""s bank account, password, and the balance of his account, if necessary, are stored in an IC card, by means of which payment is made on-line every time the card holder spends money.
It has been proposed that an enormous amount of video information handled by a digital camera be stored in a flash memory that has a capacity as large as several megabytes to 10 megabytes. A flash memory containing the video information taken by a digital camera can be connected to a personal computer for subsequent video processing. The use of a flash memory to record the video information taken by a digital camera may eliminate an additional external memory unit, such as MOs.
The widespread use of IC cards or flash memories, however, has been deterred by the need for special input/output (I/O) devices dedicated for such media. Since most commonly used I/O devices for personal computers are floppy disk drives, particularly 3.5xe2x80x3 floppy disk drives, the use of 3.5xe2x80x3 floppy disks as an I/O device for IC cards and flash memories could spur their proliferation. In fact, methods for writing and reading data into and from an IC card or flash memory are being studied, and adapters that can be inserted or incorporated in 3.5xe2x80x3 floppy disk drives have already been proposed. A conventional type of 3.5xe2x80x3 floppy disk drive has a magnetic head to read and write information from and into a 3.5xe2x80x3 diskette, and a drive shaft to cause the diskette to rotate at 300 rpm, but it has no power feeding terminals. In diskette-shaped adapters, a button battery is incorporated to power the built-in CPU. A battery, which depletes as it is used, has to be replaced with a new one every four to five months at the longest.
Introduction of a 3.5xe2x80x3 floppy disk in which a generator is incorporated and driven by the rotation of the drive shaft of a 3.5xe2x80x3 floppy disk drive would prove very useful, giving momentum to the proliferation of IC cards and similar media. In order to implement a diskette which serves as an information I/O device in conjunction with a memory card having magnetic stripes, such as an IC card, a space for housing a memory card must be provided in the diskette. The typical size of a standard memory card is 85 mm in length, 54 mm in width, and 0.8 mm in thickness. Since the thickness given above ignores the embossed part for raised letters of the memory card, its actual thickness is a little bit thicker. To provide a space for a memory card and a permanent magnet type generator in a 3.5xe2x80x3 diskette having a thickness of 3.5 mm and two 0.2 mm-thick covers on both sides, the thickness of the generator must be not more than 2.0 mm, allowing for the thickness of the memory card of 0.8 mm and some clearance for loading and unloading the card.
The thickness t allowed for the permanent-magnet generator to be incorporated in a 3.5xe2x80x3 diskette is 2.0 to 2.5 mm, as stated earlier. The size (width and length) allowed for the generator is up to the size of the 3.5xe2x80x3 diskette, or 90 mm in diagonal length d (mm) on the end face thereof. With t being 2.3 mm and d being 90 mm, the aspect ratio (t/d) is approximately 2.6%.
Commercially available motors or generators having a small thickness include spindle motors for FDD. Some of commercially available thin motors are shown in TABLE 1, where the diagonal length d, thickness t, and aspect ratio (t/d) thereof are also shown, together with the locations of gaps for the motor permanent magnets. As is evident from TABLE 1, the aspect ratio of a generator that can be incorporated in a 3.5xe2x80x3 diskette is extremely small, compared with commonly used thin motors and generators.
Incorporating a generator into a 3.5xe2x80x3 diskette has already been proposed, as in U.S. Pat. No. 5,159,182 and Published Japanese Translation of PCT International Publication for Patent Application Hei-7(1995)-500238.
U.S. Pat. No. 5,159,182 discloses that a generator is incorporated into a 3.5xe2x80x3 diskette, and that the generator has a rotor and a stator, but does not disclose any further details of its construction. In Published Japanese Translation of PCT International Publication for Patent Application Hei-7(1995)-500238, it is disclosed that a permanent magnet rotating together with a hub is provided as a generator built into a 3.5xe2x80x3 diskette; the hub having the permanent magnet is caused to rotate by the drive shaft of a floppy disk drive. It is also disclosed that the permanent magnet is of a cylindrical shape, and magnetized in the direction of the rotational shaft so that a large number of magnetic poles are provided on both end faces of the cylindrical permanent magnet. The magnetic poles on the stator yokes are arranged in such a manner as to sandwich the cylindrical permanent magnet, and a stator coil is provided between the stator yokes on both sides of the cylindrical permanent magnet.
Now, suppose a permanent magnet generator of a size that can be incorporated into a 3.5xe2x80x3 diskette, as disclosed in Published Japanese Translation of PCT International Publication for Patent Application Hei-7(1995)-500238, where a cylindrical rotor permanent magnet is magnetized in the direction of the rotational shaft so that a large number of magnetic poles are provided on both end faces of the cylindrical permanent magnet. In this case, stator magnetic poles are arranged on both sides of the end faces of the cylindrical permanent magnet via small magnetic gaps. Since the thickness allowed for the generator is 2.0 to 2.5 mm, the thickness allowed for the permanent magnet is only 0.5 to 0.8 mm. In a magnet having a short distance between the magnetic poles, even when a material having a large coercive force is used, the resulting magnet has a small magnetomotive force. Since both end faces of the cylindrical permanent magnet face stator magnetic poles, a gap of a few tenth of millimeters is needed to allow the rotor to rotate without mechanical interference with the stator magnetic poles. To ensure the gap of this size, the thickness of the stator magnetic poles must be not more than 0.5 mm, assuming that the thickness of the permanent magnet is 0.5 mm. This thickness is not enough to allow magnetic flux to pass. In Published Japanese Translation of PCT International Publication for Patent Application Hei-7(1995)-500238, furthermore, a long magnetic path is required because the stator coil is provided away from the stator magnetic poles. This, together with the use of thin stator magnetic poles, has led to an increased magnetic resistance, resulting in a generator having small output.
The present inventors already proposed a diskette incorporating a new permanent magnet generator that can be used for the above purpose, and filed an application for patent as U.S. patent application Ser. No. 09/369,420 on the filing date of Aug. 6, 1999 (European Published Patent No. EP0978930 A1, the date of publication: Feb. 9, 2000). The diskette for which the patent application was filed is shown in FIG. 13. A diskette 9 in the figure has a permanent magnet generator 90 incorporated around a hub 911 at the center thereof, and a ring-shaped permanent magnet 912 having magnetic poles on the outer circumferential surface is provided rotatably together with the hub. A generator stator 92 is provided inside the diskette on the outer periphery of the permanent magnet 912 of the rotor 91 with a magnetic gap between the magnetic poles on the outer circumferential surface of the permanent magnet 912. In FIG. 13, numeral 95 refers to a memory card housing space, 96 to a card contact terminal, 97 to an input/output terminal, 98 to CPU, and 99 to a stabilized power supply circuit, respectively.
The permanent magnet generator for which the patent application has been filed as U.S. patent application Ser. No. 09/369,420 has a rotor having magnetic poles on the outer circumferential surface of the ring-shaped permanent magnet, and stator magnetic poles arranged on the outer circumferential surface at locations facing the rotor magnetic poles, and having magnetic pole teeth extending radially outward from each of the stator magnetic poles. To increase the output of the permanent magnet generator, a permanent magnet having the largest possible coercive force and residual magnetic flux density, or more preferably an anisotropic sintered NdFeB magnet is used. The coils wound on each of the magnetic pole teeth are as many as 6,000 turns in total.
Nevertheless, the rotation of the magnet is normally as low as 300 rpm because the rotation of the floppy disk drive is used as it is to cause the rotor to rotate. As a result, the generated power of the permanent magnet is 20 mW at best despite the use of a sintered NdFeB magnet having excellent magnetic characteristics as the rotor permanent magnet.
The present inventors studied to obtain a large output, and found that the output cannot be substantially increased with the generator as disclosed in U.S. patent application Ser. No. 09/369,420 unless the revolution of the rotor is increased. This is attributed to the fact that the number of magnetic flux entering the magnetic pole teeth is small due to the small surface area of each magnetic pole of the rotor permanent magnet used, and that provision of a magnetic shield to prevent the adverse effect of the large magnetic flux leakage from the magnetic poles of the rotor permanent magnet on the surroundings tends to increase magnetic loss, resulting in reduction in the amount of effective magnetic flux.
It is therefore an object of the present invention to provide a permanent magnet generator of a thin type (aspect ratio (thickness t/diagonal length d): not more than 6%) having a large generating power that can reduce or eliminate the aforementioned drawbacks.
It is another object of the present invention to provide a thin type permanent magnet generator in which the output of the permanent magnet can be fully utilized by reducing the magnetic gap to reduce the magnetic resistance of the magnetic circuit, thereby increasing the permeance coefficient of the permanent magnet.
It is still another object of the present invention to provide a thin type permanent magnet generator that has essentially low magnetic flux leakage.
It is a further object of the present invention to provide a durable thin type permanent magnet generator.
It is a further object of the present invention to provide a diskette incorporating the aforementioned thin type permanent magnet generator.
It is a further object of the present invention to provide a diskette incorporating the thin type permanent magnet generator that can generate sufficient output with the revolution of the floppy disk drive left unchanged.
It is still a further object of the present invention to provide a diskette incorporating a thin type permanent magnet generator that has so low magnetic flux leakage that even when a memory card housing space is provided adjacent to the permanent magnet generator, the magnetic flux leaked from the permanent magnet generator does not adversely affect the memory card inserted into the space.
The thin type permanent magnet generator that can be incorporated into the diskette according to the present invention comprises
a disc-shaped rotor having a disc-shaped hub made of a soft magnetic material that is rotatable around the rotational shaft, and a flat ring-shaped permanent magnet concentrically fitted to an end face of the hub; the permanent magnet being axially magnetized in such a manner that each end face thereof has a plurality of magnetic poles having circumferentially alternately different polarities,
the magnetic poles on the one end face of the permanent magnet being magnetically short-circuited by the hub, and the magnetic poles on the other end face serving as rotor magnetic poles, and
a stator having on one end thereof each of the stator magnetic poles that can face the rotor magnetic poles at the same pole intervals via an axial gap; with a plurality of magnetic pole teeth extending radially outward from the stator magnetic poles; the magnetic pole teeth being connected to each other at the other end thereof by a yoke made of a soft magnetic material and having coils wound on the middle part thereof; and the ratio of the generator thickness/the diagonal length of the generator end face being not more than 6%.
In the above-mentioned thin type permanent magnet generator, it is desirable in preventing magnetic flux from leaking from the permanent magnet to the outside of the generator through the side of the hub outer periphery that the outer periphery of the hub protrudes more than 0.3 mm from the outer periphery of the permanent magnet. It is also desirable in preventing magnetic flux leakage from the permanent magnet to the outside of the generator through the inner opening of the stator tooth tips that the tips of the magnetic pole teeth radially inward protrude by more than 0.3 mm from the central opening of the ring-shaped permanent magnet. It is also desirable in preventing magnetic flux from leaking from the permanent magnet to the back outside of the stator magnetic teeth that the end face of the permanent magnet which end face is not covered by the magnetic teeth on the side of the stator is covered by a piece made of a soft magnetic material.
In the above-mentioned thin type permanent magnet generator, it is effective in increasing the generator output that the gap or distances between the adjacent stator magnetic poles of each of the magnetic pole teeth is 0.3 to 1.0 mm because it causes the magnetic flux of the permanent magnet to effectively cross the stator coil.
In the above-mentioned thin type permanent magnet generator, each of the magnetic pole teeth extends radially outward from the stator magnetic poles, and has a step difference in the axial direction between the portion having the stator magnetic pole and the portion on which the stator coil is wound. In other words, the magnetic pole tooth of the stator is bent so that the step difference in the axial direction is produced between the portion facing the magnet and the intermediate portion on which the stator coil is wound. Or, it is desirable that separate members be assembled in such a manner as to form a step difference. When the magnetic pole tooth is bent so as to form a step difference, it is desirable that the bent portion should have an inclined portion, with the inclined portion is inclined at an angle of 30 to 60 degrees with respect to the length direction of the entire magnetic pole teeth.
By providing a step difference on the magnetic pole tooth, a space for winding a coil is accommodated in the generator, thereby the required coil can be wound on the magnetic pole tooth without increasing the thickness of the generator. Thus, a sufficiently large generator output can be obtained.
The permanent magnet used in the present invention should preferably be a bonded magnet containing SmFeN or NdFeB magnetic powders, or a sintered NdFeB magnet, or more preferably be a bonded NdFeB magnet containing NdFeB magnetic powders.
In the thin type permanent magnet generator according to the present invention, the hub should preferably be supported by an anti-friction bearing so that the hub can be rotated around the rotational shaft. The hardness of the portion of the hub where the hub comes in contact with the balls of the anti-friction bearing should preferably be not less than HRC 35. Furthermore, the portion of the hub where the hub comes in contact with the balls of the anti-friction bearing should preferably have a U-shaped groove.
The diskette incorporating the thin type permanent magnet generator according to the present invention comprises
a diskette case having a magnetic disk shape, and
a disc-shaped rotor having a disc-shaped hub made of a soft magnetic material that is caused to rotate around a rotational shaft by an external drive mechanism, and a flat ring-shaped permanent magnet concentrically fitted to one end of the hub;
the permanent magnet being magnetized in the axial direction so that a plurality of magnetic poles having alternately different polarities in the circumferential direction are arranged on each end face of the permanent magnet: the magnetic poles on one end face of the permanent magnet being magnetically short-circuited by the hub, and the magnetic poles on the other end face of the permanent magnet operating as rotor magnetic poles, and
a stator having a plurality of stater magnetic pole teeth having on one end thereof each of a plurality of stator magnetic poles that can face the rotor magnetic poles at the same pole intervals via an axial gap, and extending radially outward from the stator magnetic poles; the magnetic pole teeth being connected at the other end to each other by a yoke made of a soft magnetic material and having stator coils wound on the intermediate portion thereof; and
the ratio of the generator thickness/diagonal length on the end face of the generator being not more than 6%.
In the above thin type permanent magnet generator, the thickness of the rotor permanent magnet in the magnetization direction should preferably be not less than 10% and not more than 30% of the thickness of the diskette, and the gap between the magnet and the stator magnetic poles should preferably be not less than 2% and not more than 15% of the thickness of the diskette, more preferably be not less than 5% and not more than 15%, and more specifically the thickness of the magnet in the magnetization direction should preferably be 0.3 to 1.0 mm, and the gap between the magnet and the stator 0.07 to 0.5 mm.
In the diskette according to the present invention, a memory card housing space should preferably be provided in the diskette case, adjoining the permanent magnet generator, and the end face of the permanent magnet generator on the side of the magnetic pole teeth having the stator magnetic poles should preferably be on the side of the memory card housing space.
That is, since a flat ring-shaped rotor permanent magnet can be used, and rotor magnetic poles are provided on one end face thereof in the thin type permanent magnet generator according to the present invention, the area of the rotor magnetic poles can be increased, and thus a large amount of magnetic flux is available. Furthermore, since all the stator magnetic poles are arranged on only one end face of the rotor permanent magnet, the thickness of the magnetic pole teeth having the stator magnetic poles can be increased, and the cross-sectional area thereof can be increased. As a result, the effective amount of magnetic flux contributing to power generation can also be increased. Moreover, since the hub made of a soft magnetic material is bonded to the other end face of the rotor permanent magnet, the permanent magnet is sandwiched between the hub and the magnetic pole teeth. This helps reduce magnetic field leakage from the permanent magnet to the outside (for example, into the memory card housing space provided adjacent to the permanent magnet generator or to the outside of the diskette case). As a result of these, the effective amount of magnetic flux contributing to power generation can be increased, and thereby a large power output can be obtained. Thus, the power generation output that has heretofore been 3 V, 20 mW can be increased to more than 5 V, 40 mW.